The present invention relates to a process and apparatus for converting solid, hydrogen deficient, ash containing, hydrocarbonaceous materials (e.g. coal) into liquid hydrocarbon materials. Particularly, the invention relates to a two stage process and apparatus for direct conversion of coal into liquid products.
It was recognized in the early 1900's in Germany that the conversion of coal to hydrocarbon fuel liquid was possible. This conversion process was referred to as coal liquefaction. Coal liquefaction may be accomplished by either direct or indirect liquefaction. Conversion of coal to liquid by the use of heat and hydrogen is designated as direct liquefaction. Conversion of coal to liquid with an intermediate step of gasification of the coal into synthesis gas (CO+H.sub.2) followed by chemical combination of the synthesis gas into liquids is termed indirect liquefaction. The present invention relates to a two stage direct liquefaction process and apparatus.
Generally, direct liquefaction describes a number of processes which include the basic steps of breaking coal into small fragments and adding hydrogen to these fragments to produce liquid fuels. One previous commercial direct liquefaction process known as the "Bergius Process" was operated on a commercial scale in Germany during World War II. The Bergius Process, however, operated at severe conditions of 10,000 psi and 900.degree. F. with an overall efficiency of about 46 percent (Btu in/Btu out). Pilot facilities operating on the Bergius principles were also built in the United States after World War II. These facilities were eventually closed down in the mid-1950's since they clearly could not compete economically with low cost crude oil.
With the advent of the current oil shortage and our present day energy crisis, economics presently favor the development of liquid hydrocarbon fuels from coal. In view of the vast coal reserves of the United States, it is evident that the development of new, improved, and economical coal liquefaction techniques will be of paramount importance in any solution to our energy problems.
At present, the direct coal liquefaction process generally comprises charging a preheater/reactor maintained at temperatures of about 750.degree.-850.degree. F. and pressures of about 1500-2000 psi with a slurry of coal, a process derived solvent and hydrogen. Under these conditions, almost all (above 75 percent) the coal dissolves and provisions are made to separate the unconverted coal and ash followed by distillation of liquid according to boiling range.
Direct coal liquefaction can be accomplished by either a one stage, or a two stage process. One stage liquefaction is defined as a process wherein the conversion from coal to distillate liquid (i.e. liquid hydrocarbon fuel) takes place in a single reactor by contacting coal, process derived solvent and hydrogen. Two stage liquefaction defines a process wherein there is a separation of the primary extraction step from the hydrocracking of the extract to distillate.
In the 1950's, a two stage liquefaction process known as the "CSF Process" was proposed. This CSF process was the first application of an expanded bed hydrocracker to clean extract resultant from the primary extraction step. The extract was cleaned by mechanical means, such as filters or a hydroclone. In addition, the CSF process did not add hydrogen in the extraction step which limited the yield of extract to about 75 percent. The results of this process were not promising (e.g. 1.83 bbl/ton with a hydrogen consumption of 6500 SCF/barrel) and the process was abandoned. Before abandonment of the CSF process an attempt was made to clean the extract by solvent deashing. However, the extraction step was still performed without the aid of hydrogen and no hydrocracking of the deashed extract was attempted.
Recent developments in coal liquefaction have concentrated on improvements in the one stage liquefaction process. At present there are three processes which have developed to a point of being considered serious candidates for commercialization: Solvent Refined Coal (SRC-I and SRC-II), H-Coal, and Exxon Donor Solvent (EDS). Each of these processes is a one stage liquefaction because (a) there is no separation of the primary extraction step from the step of hydrocracking of the extract to distillate, and (b) conversion of coal to distillate liquid takes place in a single reactor by contacting coal, process derived solvent and hydrogen.
The SRC process may be operated in two modes designated as SRC-I and SRC-II. The SRC-I process utilizes a high coal throughput feed rate during the primary extraction step followed by hydrocracking of the extract. This process produces a solid fuel resembling coal with very low ash and reduced sulfur content. The SRC-II process utilizes a low coal throughput feed rate during the primary extraction step with a recycle of the unconverted coal and heavy liquid. The SRC-II process results in a distillate fuel product part of which can be burned as fuel and the remainder refined as high octance gasoline.
The H-Coal process is derived directly from commercial H-Oil hydrocracking processes. The H-Oil process utilizes a reactor known as an ebullating bed in a catalytic process to upgrade heavy solids containing petroleum resids. The heavy solids containing petroleum resids are introduced into the catalytic ebullating bed hydrocracker resulting in the formation of a liquid fluidized bed comprising catalyst particles suspended in petroleum resids. The catalyst particles and hydrogen present in the ebullating bed hydrocracker upgrade the petroleum resid to a lighter liquid product. The heavy solids present in the petroleum resids deposit out onto the catalyst surface. Due to the unique design of the ebullating bed, the liquid product is recirculated through the catalyst particles establishing a flushing action which tends to wash some of the solids off the catalyst particles reducing deactivation and increasing catalyst lifetime. This H-Oil process has been applied directly to hydroextracted liquid coal without removal of ash or unconverted coal.
The Exxon Donor Solvent (EDS) Process applies known commercial petroleum processing steps to coal conversion. It was discovered that hydrotreating the process solvent by passing the solvent over an active catalyst to obtain a predetermined hydrogen content produced an enhanced process solvent. This enhanced process solvent could be used to dissolve and hydrocrack coal in the presence of hydrogen without the use of a catalyst. The net effect of the use of the more effective solvent is that only clean, distillate process solvent contacts catalyst resulting in minimal catalyst fouling and total distillate fuel yields comparable to those obtained by the SRC-II and H-Coal processes.
While the above described one stage processes represent some improvement over the earlier Bergius and CSF processes, they possess the disadvantages of relatively low yield of liquid hydrocarbon product and too high consumption of hydrogen.